142 



THE VITAL PROCESSES 



and grinding action of the teeth, to a finely divided condi- 

 tion. By the second, the saliva becomes mixed with the 

 food and is made to act upon it. 



Accessory Organs of the Mouth. The work of mastica- 

 tion and insalivation is accomplished through organs situ- 

 ated in and around the mouth cavity. These comprise : 



i. The Teeth. The teeth are set in the upper and 

 lower jaws, one row directly over the other, with their 



hardened sur- 

 faces facing. In 

 reducing the food, 

 the teeth of the 

 lower jaw move 

 against those of 

 the upper, while 

 the food is held 

 by the tongue and 

 cheeks between 

 the grinding sur- 

 faces. The front 

 teeth are thin and 

 FIG. 65. The teeth. A. Section of a single chisel-shaped, 

 molar, i. Pulp. 2. Dentine. 3. Enamel. 4. Crown. They do not mee t 

 5. Neck. 6. Root. B. Teeth in position in lower jaw. 



1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. so squarely as do 



C. Upper and lower teeth on one side. I. Incisors, the back ones, but 



2. Canines. 3. Bicuspids. 4. Molars. 5. Wisdom, their edges glide 



D. Upper and lower incisor, to show gliding contact. , , 



over each other, 



like the blades of scissors a condition that adapts them 

 to cutting off and separating the food (D, Fig. 65). The 

 back teeth are broad and irregular, having surfaces that 

 are adapted to crushing and grinding. 



Each tooth is composed mainly of a bone-like substance, called 

 dentine, which surrounds a central space, containing blood vessels and 



D 



